Method of heating tar



Dec. 15, 1936. s. P. MILLER 2,064,486

MTHOD OF HEATING TAR Filed Dec. 1, 192s 2 sheets-sheet 1 ATTORNEWvul/11111111111111111111'l'lafalzflllzrW/h Dec.. 15, 1936. s, P, WLLER2,064,486

METHOD OF HEATING TAR Filed' Dec. l, 1928 2 Sheets-Sheet 2 7*-0co/voj/E/ -1/ 33 f3 INVENTOR ATTORNEY Ptented Dec. l5, 1936 STATESArr-Nr OFFICE xvm'rHoD oF HEATING TAR Application December 1, 1928,Serial No. 323,063

2 Claims.

This invention relates to the distillation of organic materials such ashydrocarbons including tar, tar oils, petroleum, petroleum oils and thelike, and particularly to a method whereby distillation may be conductedwithout diiculties due to carbonization resulting from decomposition ofthe material.

In distilling materlaL-e. g. tar,-in continuous stills, the material isordinarily pumped through tubes exposed to heat from combustion gases.'I'he material flows counter-current to the heating gases and commonlybecomes highly superheated. The superheated tar is allowed to ow into achamber known as a flash box or vapor box in which the oil vapors escapefrom the superheated tar and pass to suitable condensers. The residueremaining in the flash box is commonly withdrawn continuously. In thecase of tar, distillation may be conducted in one, two or more stages.If two stage distillation is-carried out the tar is moderately heated intubes. positioned in the cooler section of the flues. I'he oil vaporsareallowed to flash olf in a vapor box and the residual pitch is returnedto tubes in the hotter section of the furnace for completion of theheating.

In either the single or multiple stage systems tar or pitch is highlysuperheated by indirect exposure to progressively hotter gases. Further,not only is the very hottest tar or pitch usually exposed to the hottestgases but it isalso exposed to radiant energy from the source of heat orcombustion zone, i. e. the heating tubes through which the hottest taror pitch is flowing are directly exposed to the flame of the combustiongases and to the high temperature side walls of the furnace.

When tar is being distilled to pitches of low or medium melting point,e.g. up to 160 F.,- little diilculty due to carbonization `of the materialflowing through the heating 4tubes is experien'ced due to the fact thatonly a relatively low temperature of tar or pitch is required fordistillation and furnace temperatures may correspondingly be lowered.However, when pitches of higher melting point are produced, it isnecessary to maintain high furnace temperatures in order to attain therequired temperature of preheat of the tar or pitch in the tubes.Carbonization in the tubes exposed to radiant energy from the furnace isexperienced. Several factors are responsible for carbonization. Tar orpitch contains materials readily decomposable by contact with surfacesat high temperature,-e. g., temperatures of 700 F. and upwards.

Tar and pitch have relatively low unit heat absorption capacity, theirspecific heat being only 0.3 to 0.45 and the latent heat of the volatileoils distilled oi is only approximately 150 B. t. u. per pound.

At furnace temperatures prevailing during the distillation of tar toproduce hard pitch, the rate of heat input to the tubes due to radiantenergy is of the order of ten times the rate of heat input due toconvected heat.

At the high temperatures prevailing, extreme local overheating of thetar or pitch results due to its low unit heat absorption capacity, andserious decomposition with attendant carbonization on surfaces of thedistilling vessel or heatingtubes results. 'I'he deposit of carbon stillfurther reduces the capacity of the system to absorb heat. Higherfurnace temperatures .become necessary and tubes finally burn out due toextreme furnace temperatures and excessive carbon deposits.

During the period when carbon is built up in the tubes, distillationcapacity of the system is gradually reduced. A serious loss of valuabledistillate is experienced due to decomposition. The difculty experiencedis due to absorption of radiant energy at high temperatures via thewalls of the distilling vessel or tubes by the tar or pitch thereinwhich is already near the decomposition temperature.

Very little diiculty is experienced from convected heat at hightemperatures due to the relatively low rate at which it can be absorbedat the heating surfaces. The tar or pitch being distilled ordinarily hasadequate capacity for absorption of yconvected heat and hence littlecarbonization of heating surfaces results therefrom.

It is the object of the present invention to shield the materialundergoing distillation from the radiant heat and to permit, therefore,a more uniform and satisfactory transfer of heat to the material to bedistilled. Decomposition and especially the formation of carbon depositsare avoided, the ediciency of the apparatus is increased, and its lifeis prolonged with consequent reduction of cost.

I have discovered that the above difficulties may be overcome and theshielding of the material from radient heat accomplished by interposinga suitable fluid medium between the source of heat and the tubes orvessel containing the material undergoing distillation so as to absorbthe radiant energy and thus screen the material from the heating effectthereof. The heat absorbed in the fluid may be applied usefully inheating the material undergoing distillation or otherwise. In

the practical application of the invention, the fluid may be gaseous orliquid. A suitable gaseous medium is flue gases discharged from theheating apparatus, a portion of which may be recirculated, caused toabsorb the radiant energy, and thereafter to give up heat to thematerial undergoing distillation. Various liquids may be employed in asimilar way. Water, for example, may be circulated in the apparatus insuch a way as to absorb the radiant energy, the water being therebyconverted into steam which can be utilized in or about the plant for anydesired purpose. Similarly a metallic alloy which is iluid at thetemperature to which it is subi ected may be circulated so as to absorbthe radiant energy and may be utilized thereafter as a heating mediumfor the material undergoing distillation. Likewise liquid of a charactersimilar to that undergoing distillation may be employed to absorb theradiant energy and may be combined thereafter with the materialundergoing distillation so as to supply heat thereto.

If the material to be distilled is used for absorption of radiantenergy, it will be best to pass a portion of the material directlythrough the tubes exposed to radiant heat without previous heating byexposure to counter-current flow of the hot gases.

The tar or other material at relatively low temperatures will have acapacity for absorption of heat considerably in excess of that of thehighly heated'tar or pitch in the usual system. Carbonization in thetubes and losses of oil due to decomposition will be avoided. 'Iheheated tar leaving the tubes exposed to radiant energy may be combinedthereafter with the remainder of the tar exposed substantially only toconvected heat and will preferably, although not necessarily, becombined at a point where the temperatures of the two streams ofmaterial are approximately the same.

It is to be understood that the primary purpose of the invention is theinterposition of a screen between the source of heat and the materialundergoing distillation so as to avoid the effect of radiant energy, andthat various fluids, including those mentioned, may be employed inapparatus adapted for the accomplishment of the purpose. A suitableapparatus involves a series of tubes through which the material to bedistilled is circulated and which are surrounded by hot gases derived,for example, from a furnace which is supplied with fuel and airsuiiicient to maintain combustion. In such an apparatus a series oftubes adjacent the source of heat and exposed to radiant energytherefrom may be utilized as the screen, and in such tubes the fluidshereinbefore mentioned may be circulated. The utilization of such fluidsafter they have absorbed heat in the tubes will vary depending upon thecharacter thereof. Thus if ilue or other gases are utilized, such gases,after heating, may be delivered to the furnace gases and, minglingtherewith, will circulate about the remaining tubes in which thematerial undergoing distillation is subjected to heat. The gasescirculating through the initial series of tubes absorb the radiantenergy, and the heat is thereafter distributed and utilized indistilling material in the remaining tubes, the effect of distributionbeing, however, to avoid overheating of the material in any of thetubes. If water is utilized in the screen,

^ steam will be formed and may be employed as a sc-rce of power or forany other purpose to which it is adapted, the heat being therebyutilized and spouse the material undergoing distillation being protectedfrom the radiant energy by absorption thereof in the water. If a moltenmetallic alloy is used, it may be circulated throughthe initial seriesof tubes and thereafter through the jackets of successive tubes in whichthe material undergoing distillation is subjected to heat transmittedfrom the molten, alloy. The radiant energy is absorbed by the alloy andis distributed through the jacketed tubes tothe material undergoingdistillation, and overheating of the latter is avoided. If materialsimilar to that undergoing distillation .is utilized in the tubes of theinitial A theobject being attained nevertheless. It is possible,consequently, to distill materials such as those mentioned en'ectivelyand economically and without decomposition such as normally causesexcessive deposits of free carbon in the apparatus.

The invention will be more readily understood by reference to theaccompanying drawings and the following description. The drawings arediagrammatic and are merely illustrative of apparatus suitable for thepractice of the invention. In the drawings- Fig. l is a sectional viewof a still adapted to permit protection of the material by circulationof a gaseous medium;

Fig. 2 is a similar view of a still adapted for the utilization of waterto absorb the radiant energy; l

Fig. 3 is a sectional view of a still in which the radiant energy isabsorbed in a circulating metallic alloy;

Fig. 4 is an enlarged sectional view of a jacketed tube for use inconnection with the still illustrated in Fig, 3;

Fig. 5is a sectional view, taken at right angles to the showing of Fig.3, of a still in which the material to be distilled is itself employedto afford the screen: and

Fig. 6 is a sectional view, taken at right angles to the showing of Fig.3, of a still in which the material to be distilled is employed toafford a screen.

Referring to the drawings, 5 indicates a suit- `able enclosure of brickor other refractory material forming the distillation chamber. Pipecoils 6 and 'l are disposed in the chamber and are adapted to receiveand to convey the material undergoing distillation through the chamber.It is to be understood that the material is supplied from any suitablesource and that the products of distillation are conducted to condensers(not shown) wherein the desired fractions are separated and recovered.Hot gases are supplied by a furnace 8 in which any combustible materialmay be burned with an adequate supply of air. A plurality of tubes 9 areconnected to headers in, the tubes being disposed at the end of thedistillation chamber adjacent the furnace, so that the tubes aresubjected to radiant energy therefrom. The gases from the furnace passthe tubes 9 and circulate about the tubes 6 and 'I to effect the desiredheating of the material to be distilled. The gases escape to a stack I ithrough which they are discharged to the atmosphere. A portion of thegases may be returned through a pipe I2 and blower I3 to the tubes 9.Circulating through I the tubes, the stack gases absorb radiant energyand the heat thereby transmitted to the gases is returned to thedistillation chamber byconveying the gases'through the pipe I4 whichdischarges into the chamber. The gases thus mingle with the combustiongases and circulate around tubes 6 and 1, thus heating the material inthe tubes.

In Fig. 2, the apparatus is modified by providing a chamber I5 of brickor other suitable material enclosing the tubes I6 in which the materialto be heated is circulated. A furnace I1 is adapted to supply combustiongases, and these gases, after circulating through the distillationchamber, escape to a stack I8. A p1urality of tubes I9 are disposed inthe distillation chamber adjacent the furnace so as to be subjected tothe radiant heat therefrom. Water is supplied to the tubes I9 from asource thereof, and circulates through the tubes. being thus subjectedto the radiant heat. The water is heated and converted into steam, andthe steam is conveyed through a pipe 20' to a steam drum 20 from whichit may be distributed and utilized. The water may be circulated at sucha rate that substantially no steam is formed, the water being merelyheated. It may be conveyed thereafter to a boiler (not shown) as"make-up. 'I'he steam produced in the boiler may be utilized for anypurpose. In this embodiment of the invention the material to bedistilled is screened eiectively from the radiant heat, and the heat isutilized in an economical manner.

In Figs. 3 and 4 I have illustrated an apparatus adapted particularlyfor the utilization of a molten metallic alloy as a heat absorber. Inthis apparatus a chamber 2|, preferably constructed of ilreproof brick,is provided with tubes 22 and 23. The tubes 22 are disposed adjacent theoutlet 24 from a furnace 25 which may be supplied with fuel and airsufficient to afford combustion gases at. high temperature. The gasespass through the chamber 2l about the tubes therein and thence to astack 26.

The tubes 22 are supplied with a molten metallic alloy, forI example oneconsisting principally of lead. The alloy is circulated through thetubes and absorbs the radiant heat from the furnace.

The tubes 23, as illustrated in Fig. 4, consist of jackets 21 whichsurround the tubes 23. The latter are connected and are adapted topermit the circulation of the material to be distilled. The moltenmetallic alloy first heated in the tubes 22 is circulated by a pump 29through the jackets 21 and is thus permitted to give up its heat to thematerial undergoing distillation. 'Ihe partially cooled alloy is thenreturned to the tubes 22 and vabsorbs additional heat therein. Thematerial undergoing distillation is thus heated uniformly to the desiredtemperature, andoverheating is avoided, the tubes 22 forming aneffective screen for the radiant heat.

In Fig. 5, the apparatus is similar to that illustrated in Fig. 3, thedistillation chamber 30 being connected to a combustion chamber (notshown) in which combustion gases are produced from the fuel and airsupplied thereto. After passing through the distillation chamber thecombustion gases escape to a stack 33. Banks of tubes 34 and 35 areprovided in the distillation chamber. As indicated diagrammatically inFig. 5, banks of tubes 34 and 35 are connected in such a way as willallow one or more rows of bank of tubes 34 to be cut oi from bank 34 andadded to bank 35, thus allowing control of the tube surface used asradiation shield. The tubes are connected as shown somewhatdiagrammatically at 36, 31 and 38. If all four rows of tubes in bank'34are to be used as radiation shield, valve 39 will be open,.40 closed, 4Iopen, 39 closed, 40 open and 4I closed. By suitably manipulating Lthevalves, as will be understood by one skilled in this art, the materialmay be delivered through tubes 34 preheated by the radiant energy fromthe heating source, then passed by connection 42 or 43 into and throughtubes 35 where fresh material `may lbe added thereto.

Using all four rows of tubes in bank 34 as a radiation shield, the taris introduced at 4l, passed through the four rows of tubes in bank 34,thence into and through bank 35 at 42 or 43, where the heated tarmingles with the tar passing through the bank of tubes 35, the resultantmixture leaving the still through valve-controlled conduit 39.vEmploying three rows of tubes in bank 34 as the radiation shield, thetar is introduced through 4I', passed through the three lower rows oftubes in bank 34, .then through connections 42 or'43 where the heatedtar mingles with the tar passing through the bank of tubes 35, themixture passing through the four lower rows of tubes in bank 35, and ifdesired, into and through the upper row of tubes in bank 34, leaving thestill at 39. If only two rowsr of tubes in bank 34 are used as theradiation shield, the tar is introduced through 4 I passes through thetwo lower rows of tubes in bank 34, thence into the bank 35 at either 42or 43 where it mingles with the tar passing through the bankiof tubes35. The mixture may leave the bank of tubes 35 at 39 or be passedthrough the two upper rows of tubes in bank 34, leaving the still at39".`

By regulating the flow of material through tubes 34 and by causing thematerial to pass through one or more banks of tubes 34 the amount ofpreheat imparted to the material passing through the tubes 34 may becontrolled at will.

Preferably in the distillation of coal tar the tar is preheated bypassage through a definite number of rows of tubes in bank 34,-e. g.,the 2 rows adjacent to the furnace,-and is then heated to vaporize thelighter oils by passage through tubes in bank 35, the resultantsuperheated tar being passed into a flash-box or vapor box in which theoil vapors escape. 'I'he resultant pitch lis then passed through theremaining rows of tubes in bank 34 and after being heated by absorptionof both the residual radiant energy and by convected heat, is withdrawnfrom the still. Referring to Fig. 6, tar is introduced into theradiation shield through valve-controlled pipe 5I and passes through thetwo lower rows of tubes in bank 34, thence through pipe 5,2, asindicated by the arrows, into either valvecontrolled pipe 53 or 54.Fresh tar is introducedY diately therebelow, and leaving the stillthroughA the valve-controlled line 6I. The flow of pitch through tubesin bank 34 is controlled to prevent decomposition of the pitch thereinand to 4 l i obtain the desired heavier oils. It will be understood thatthe material supplied to the tubes which form the radiant energy screenis of such a character, is in such condition, or is-circulated beaccomplished by regulating the now of thev medium through the tubesexposed to radiant heat by control of valves shown, so that all or any.desired portion of the radiant energy isabsorbed by the medium passingthrough the tubes.

The several adaptations of the invention as hereinbefore describedinvolve the same principle, to wit, the control of decomposition andcarbonization in or on the heating surfaces by the absorption of all ora portion of the radiant energy to which the products being distilledhave ordinarily been exposed while at maximum temperature, and theabsorption or removal of the excess radiant energy in a fluid mediumother than the most highly heated portion oi' the product undergoingdistillation.

'I'he heat may b e supplied by combustion or otherwise, for example byelectric heaters. In either case absorption of radiant energy bymaterial other than that which has already been heated to a hightemperature is effected to avoid undesirable decomposition and thedeposition of carbon in the tubes. The liie oi the tubes is therebyprolonged, and the emciency thereof is maintained, so that the apparatusmay operate continuously and economically. The invention is applicableto the distillation of tar particularly, but it may be used also indistilling other organic liquids, e. g. petroleum oil, or fractions orresidues thereof.

Various changes,r may be made in the procedure and particularly in theapparatus employed without departing i'rom the invention or sacriilcingany of the advantages thereof.

I claim:

1. In a method of distilling coal tar the steps which comprisegenerating combustion gases, passing said combustion gases through azona" where they ilow in indirect'heat exchange with molten metal,whereby said molten m'etal acts as l0 a radiant heat screen, passingmolten metal from said rst zone in a path surrounding the tar beingdistilled in a second zone while passing said combustion gases from saidilrst zone into indirect heat exchange relation with said molten metall5 in said second zone, passing said combustion gases from said secondzone into indirect heat exchange relation, without the interposition oi'molten metal, with said tar in a third zone, and passing said tar fromsaid third zone through the path N surrounded by said molten metal insaid second zone.

2. In a method of distilling coal tar the steps which comprisegenerating combustion gases,

passing said combustion gases through'a zone 'where they ow in indirectheat exchange with molten metal, whereby said molten metal acts as aradiant heat screen, passing molten metal from said ilrst zone in a pathsurrounding the tar being distilled in a second zone while passing nsaid combusition gases from said rst zone into indirect heat exchangerelation with said molten metal in said second zone, passing saidcombustion gases from said second zone into indirect heat exchangerelation, without the interposition oi' molten metal, with said tar in athird zone,pass ing said tar from said third zone through the pathsurrounded by said molten metal in said second zone and recycling saidmolten metal thrdugh said rst zone.

STUART PARMELEE MILLER.

